Using the underlying method you can access the individual pixel values of images in emgucv and then find the average.
For color images EmguCV stores Red, Green and Blue Data in layer 0,1,2 of the image respectively
for (int i = 0; i < img1.Width; i++)
{
for (int j = 0; j < img1.Height; j++)
{
img4.Data[i,j,0] = (img1.Data[i,j,0] + img2.Data[i,j,0] +img3.Data[i,j,0])/3;
img4.Data[i,j,1] = (img1.Data[i,j,1] + img2.Data[i,j,1]+ img3.Data[i,j,1])/3;
img4.Data[i,j,2] = (img1.Data[i,j,2] + img2.Data[i,j,2]+ img3.Data[i,j,2])/3;
}
}
Note: The above code assumes that all the images are of equal size(height and width),if its not the case you should iterate for the image with the biggest size, and set the unavailable images's(or smaller image's) corresponding pixels to zero before adding .
You should also use some thing as Math.Floor
or similar to normalize each pixel values as pixel values can't be in decimals.